Colloquium: The Cosmic Dipole Anomaly

TL;DR

The paper surveys the cosmic dipole anomaly as a critical test of the Cosmological Principle and ΛCDM, focusing on the Ellis & Baldwin test which predicts a kinematic matter dipole with amplitude different from the CMB dipole by a factor set by the local source counts slope and spectral index. It details the SR and GR derivations, practical measurement considerations, and the history of detections, including independent confirmations that reveal a dipole larger than the kinematic expectation by roughly a factor of two, with consistent direction. The work also analyzes uncertainties, systematics, and potential astrophysical or new-physics explanations, concluding that the anomaly poses a serious challenge to FLRW cosmology and motivates upcoming large, multi-wavelength surveys (e.g., SPHEREx, Euclid, Rubin/LSST, SKA) to resolve the tension. Overall, the article emphasizes the need for careful handling of flux limits, biases, and redshift tomography to determine whether the dipole anomaly signals new physics or unrecognized systematics within the standard model. The findings have significant implications for our understanding of cosmic isotropy, the nature of large-scale structure, and the validity of the standard cosmological framework.

Abstract

The Cosmological Principle, which states that the Universe is homogeneous and isotropic (when averaged on large scales), is the foundational assumption of Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmologies such as the current standard Lambda-Cold-Dark-Matter (ΛCDM) model. This simplification yields an exact solution to the Einstein field equations that relates space and time through a single time-dependent scale factor, which defines cosmological observables such as the Hubble parameter and the cosmological redshift. The validity of the Cosmological Principle, which underpins modern cosmology, can now be rigorously tested with the advent of large, nearly all-sky catalogs of radio galaxies and quasars. Surprisingly, the dipole anisotropy in the large-scale distribution of matter is found to be inconsistent with the expectation from kinematic aberration and Doppler boosting effects in a perturbed FLRW universe, which is the standard interpretation of the observed dipole in the cosmic microwave background (CMB). Although the matter dipole agrees in direction with that of the CMB dipole, it is anomalously larger, demonstrating that either the rest frames in which matter and radiation appear isotropic are not the same, or that there is an unexpected intrinsic anisotropy in at least one of them. This discrepancy now exceeds 5σ in significance. We review these recent findings, as well as the potential biases, systematic issues, and alternate interpretations that have been suggested to help alleviate the tension. We conclude that the cosmic dipole anomaly poses a serious challenge to FLRW cosmology, and the standard ΛCDM model in particular, as an adequate description of our Universe.

Figures (5)

• Figure 1: Quasar spectral flux densities at flux limit for the sample used by Secrest:2022uvx within the WISE W1 passband, shown at bottom for reference. The fainter lines are individual quasars while the solid black line is the mean. The dotted line shows a power law with $\alpha=1.06$ used by Secrest:2022uvx, derived from the $W1-W2$ color. While the individual quasars have varying spectral indices and some show emission features, on average the flux density within the passband is close to a power law.
• Figure 2: Citations to 1984MNRAS.206..377E as of September 2025, from the https://ui.adsabs.harvard.edu/abs/1984MNRAS.206..377E/abstract.
• Figure 3: TGSS, WENSS, SUMSS, and NVSS dipole amplitudes from Siewert:2020krp, compared with the WISE dipole amplitude from Secrest:2020hasSecrest:2022uvx. The dotted line indicates the functional frequency dependence proposed by Siewert:2020krp using only the radio data, which is however not consistent with the WISE infrared value. The dashed line and shaded interval show weighted arithmetic mean and standard error, excluding the TGSS data point, demonstrating that the data are otherwise generally consistent with no frequency dependence of the dipole. The grey hatching indicates the standard kinematic expectation for $1 < x < 2$ and $0.5 < \alpha < 1.5$.
• Figure 4: Overview of the most significant matter dipole measurements to-date using the CatWISE AGN, NVSS, and RACS-low samples Secrest:2022uvxWagenveld:2025ewl. Top panel: Contours enclosing 90% of the posterior probability of the inferred dipole directions plotted onto a smoothed version of the CatWISE AGN number counts. The star marks the CMB dipole direction. Bottom left panel: Posterior probability densities of the inferred dipole amplitudes $\mathcal{D}_\mathrm{obs}$ compared against their standard expectations, viz. the expected kinematic matter dipole amplitudes $\mathcal{D}_{\rm kin}$ computed for the respective samples. Bottom right panel: Same as left panel but scaled by the respective kinematic expectations.
• Figure 5: The variation with flux cut of the dipole amplitude and its direction for WISE-selected quasars; the dotted lines show the kinematic expectation corresponding to the CMB dipole. It is seen that the amplitude remains stable (and about twice as high as the CMB-based expectation) while the direction remains close to that of the CMB dipole. The light and dark blue regions indicate the 5--95 and 16--84 percentiles of the posteriors after marginalizing over all other parameters, while the solid black line shows the median.